Who would have thought that a comic book based on Batman: The Animated Series would ever become one of the most valuable modern comics?

Selling for up to $2,000, Batman Adventures #12 is the first comic book appearance of Harley Quinn. In this issue, Barbara Gordon is terrorized by a supervillain team-up of Harley Quinn, Poison Ivy, and Catwoman.

Before Batman Adventures #12, Harley Quinn’s actual first appearance in the DC Universe was in the 1992 TV show Batman: The Animated Series episode “Joker’s Favor”.

Batman: Harley Quinn contains a piece of the “No Man’s Land” storyline, taking place after a massive earthquake leaves Gotham city crippled.

This graphic novel tweaks some of Harley Quinn’s origins, and for continuity purposes, this is how Harley Quinn gets her start as a villain. For those just getting into Harley Quinn comics, this is a great place to begin.

The iconic Batman: Harley Quinn cover art, masterfully painted by Alex Ross, is alone worth the cost of picking up this issue.

Harley Quinn #1 (2000)Harley’s solo debut!

Giving a side character a full solo series is a risky bet – one DC was willing to take with 2000’s Harley Quinn #1.

Telling Harley Quinn’s continued obsession with Joker and her further descent into madness, Harley Quinn #1 is a great modern comic. Also, this could be an undervalued issue and overall, Harley Quinn comics have a lot of collector’s potential.

In total, there are 38 volume 1 Harley Quinn comics, with the series coming to an end in 2004 when Harley is locked away in Arkham Asylum. The run is very collectible.

Who would have thought that Superman’s pal Jimmy Olsen would team up with Harley Quinn?

Taking place during the alien invasion storyline Our Worlds at War (and presumably during Harley Quinn’s solo series), Harley helps Jimmy Olsen make it back to Metropolis.

While not incredibly important to Harley’s background, Harley Quinn: Our Worlds at War shows her "soft" caring side and slow descent into anti-hero vigilantism. For collectors of Harley Quinn comics, this is an affordable title to pick up.

Harley & Ivy: Love on the Lam #1 (2001)

Just two girls out having fun, Harley & Ivy: Love on the Lam #1 documents an incredible night had by Harley Quinn and her best frenemy Poison Ivy.

Again, while not very significant to Harley Quinn’s overall story, Harley & Ivy is a hard to find one-shot. If you’re into obscure comics, collectible modern comics, or you’re just a Harley Quinn comics completionist, Harley & Ivy: Love on the Lam deserves to be on your shelf.

Gotham Girls #3 (2002)Super-Hot Issue; Classic Cover

Based on the animated show of the same name, Gotham Girls was a five-part comic series that starred Harley Quinn, Poison Ivy, Catwoman, Batgirl, and Renee Montoya. GothamGirls #3 is specifically dedicated to Harley Quinn.

Set in the Batman Animated Universe, GothamGirls #3 is intrinsically collectible because of its obscurity and Harley Quinn-centered story.

Batman Adventures 16 (2004)Harley Quinn marries Joker!

Say what you want about Joker and Harley Quinn’s relationship, but you’ve got to admit that it has spark.

Published a few months after her solo title ended, Batman Adventures #16 (vol. 2) tells the tale of Joker and Harley Quinn’s epic wedding. Batman Adventures #16 is set in the Batman Animated Universe, so it doesn’t really tie in with the rest of the Harley Quinn comics.

Batman Adventures #16 is the most valuable comic of Batman Adventures volume 2, only because of this historic wedding, and is currently a hot comic.

Gotham City Sirens #1 (2009)Rare, mature, sexy, and important to Harley's development

After Batman's "death" (yeah, right), Harley and her girl squad are rich and doing whatever they want in Gotham City. Gotham City Sirens #1 was a female-driven series that puts the villains first. Several issues were written by Paul Dini (Harley's creator).

After Gotham City Sirens ended (and all of DC's other titles), the New 52 was rolled out and Harley Quinn was welcomed into the Suicide Squad. In terms of DC cannon and continuity, this is the first Suicide Squad ever.

The New 52 relaunch marks Harley Quinn's redesign, ditching her classic jester outfit for an edgier and sexier costume. Her hair is dyed red and black – no more silly cap.

Since the Suicide Squad movie is the first time many have heard of Harley Quinn, Suicide Squad #1 is becoming a very valuable modern comic.

Harley Quinn V.2 #1 (2014)Harley's second solo series

Further revamping Harley Quinn for modern comics, this 2014 solo series portrays Harley as an anti-hero vigilante. She has distanced herself from Batman and Joker, instead living on Coney Island and working as a psychologist.

Since its launch, Harley Quinn has been well received and is notably different from other New 52 titles. The 2015 Harley Quinn Annual #1 features scratch-and-sniff panels.

New Suicide Squad #1 (2014)New-Look Squad, Movie Coming

Still a valuable member of the world’s most dysfunctional team, Harley Quinn continues to operate as an unhinged vigilante.

New Suicide Squad picks up where the 2011 series ended. In their new role, the squad is detailed to various parts of the world to fight militarized superheroes, former Suicide Squad members, and unravel the conspiracy behind the team’s creation.